Method for depositing a corrosion-protection coating

ABSTRACT

The use of a cement in a process for pack cementation deposition on a substrate having cavities of minimum equivalent diameter e cm , characterized in that the cement consists of spherical particles each having a diameter d such that d≦e cm /10.

The present invention relates to the production of a corrosion-protection coating on a substrate that has cavities.

The techniques for producing coatings may be grouped into three large families:

thermal spraying,

chemical vapor deposition, and

physical vapor deposition.

Thermal spraying techniques such as plasma or flame spraying consist in sending molten or partially molten particles, at high velocity, to the surface of the part to be protected. The coating is constructed of successive layers. These techniques can only be used on open or readily accessible surfaces.

The vapor deposition techniques use a gaseous precursor of the coating to be produced. This precursor may be produced in direct proximity to the surface to be coated (pack cementation) or be transported via a gas to the surface to be coated (out of pack, CVD using a gas cylinder or mixture, . . . ). The main difficulties encountered for pack cementation are linked to the filling of parts that have a complex geometry or very small dimensions (several mm) with the cement powder (precursor mixture of the coating). The main limitations of techniques that use gaseous precursors relate to the rapid depletion of reactive species from the gaseous mixture leading to heterogeneities of chemical composition and/or of thickness of the coating. It is very difficult to obtain a uniform coating on large surfaces or in complex geometries.

The physical vapor deposition techniques consist in evaporating the constituent element(s) of the coating before condensing them on the surface of the part to be coated. The evaporation generally takes place by bombarding a target with a high-energy (electron or ion) beam. The distance between the target and the surface to be coated is a major parameter for the uniformity of the thickness of the deposition. These techniques are very difficult to use on parts of complex geometry or on inaccessible surfaces.

The intensification of industrial processes leads to materials being used under increasingly harsh conditions and to the size of the parts used being reduced. In most cases, it is necessary to protect the parts from their surroundings with a coating. As presented in the preceding paragraphs, complex geometries and inaccessible surfaces pose problems for producing coatings using conventional techniques.

It is therefore necessary to develop new deposition techniques or to adapt the existing techniques to the new constraints.

Pack cementation is a very old process for producing a coating on a part. The latter is placed in a bed of cement powder, which is a mixture of products capable of generating a reactive atmosphere at high temperature. This cement must be placed in the vicinity of the surface to be coated in order to produce a coating that is uniform in terms of thickness and chemical composition. Coatings are conventionally produced on parts having cavities of several centimeters by filling the part with the cement powder.

However, when the cavities have characteristic sizes of the order of a millimeter, and high aspect ratios (length/width ratios), introducing the cement is much more complex. This is why processes using powder, of pack cementation type, are generally used for parts that have no or few zones that are difficult to access.

Hence, one problem that is faced is to improve the pack cementation deposition processes in order to enable the use thereof for coating a substrate that has cavities.

One solution of the present invention consists of the use of a cement in a process for pack cementation deposition on a substrate having cavities of minimum equivalent diameter e_(cm), characterized in that the cement consists of spherical particles each having a diameter d such that d≦e_(cm)/10.

The size of the cement particles may be measured by laser particle size analysis or with the aid of a screen in order to ensure that no cement particle or agglomerate of cement particles exceeds the required maximum size.

A step of deagglomeration may be necessary in order to “break” the agglomerates of individual particles that may exceed the required maximum size.

The equivalent diameters of the particles are conventionally between 1 μm and 1 mm, preferably between 1 μm and 100 μm.

The equivalent diameter is defined as the diameter of the cylinder or of the circle that is inscribed within the smallest cross section giving access to the surface to be coated. Specifically, the latter does not necessarily have a standard shape.

Depending on the case, the use according to the invention may have one or more of the following features:

the cement consists of spherical particles each having a diameter d such that d≦e_(cm)/10;

the cement comprises a precursor of the element to be deposited, an activating agent and an inert diluent;

the cement comprises 10% to 60% of metallic powder as precursor of the element to be deposited, 5% to 40% of activating agent, and a balance to 100% of inert diluent, the inert diluent preferably comprising refractory oxides;

the metallic powder consists of aluminum or a mixture of aluminum with Ni_(x)Al_(y) or Al_(x),Cr_(y), particles;

the cement comprises a precursor of the element to be deposited, a pickling flux and an inert diluent;

the cement comprises an organic or inorganic binder. The organic binder may be PVA (polyvinyl acetate) and the inorganic binder may be SiO₂. Specifically, the organic or inorganic binder may be used during a step of atomization of the powder mixture. This optional step makes it possible to improve the flowability of the powder and therefore the filling of the part. It is a matter of forming spherical agglomerates of the powder mixture. This step will preferably be carried out under an inert atmosphere in order to avoid surface oxidation of the metallic powders which may be detrimental to the deposition.

The inert compound is not chemically involved in the formation of the coating. Its main role is to avoid the densification of the cement which would prevent the elimination thereof after deposition. In general, it is a highly stable refractory compound. The content thereof is the balance of the other two compounds.

The solution according to the invention enables the production of a pack cementation deposition on parts of complex geometry and in cavities that are difficult to access.

The cement used within the context of the invention has a very good flowability that makes it possible to fill the smallest interstices (of diameter <1 mm) and to be uniformly distributed inside the whole of the cavity to be coated. The particle size distribution and the morphology of the cement particles are the principal parameters for ensuring a good flowability of the mixture.

The particle size distribution is adjusted as a function of the equivalent diameter of the smallest passage of the cavity. Regarding the morphology, the spherical shapes which may be obtained by various techniques of grinding the powders or the powder mixture are also favored. An atomization treatment of the powder mixture could also be used to form spheres of the powder mixture. In the latter case, organic additives could be used to ensure a good cohesion of the spheres and a uniform dispersion of the elements of the mixture.

The present invention also relates to two processes for depositing a coating by pack cementation on a substrate having cavities of minimum equivalent diameter e_(cm).

The first process for depositing a coating by pack cementation on a substrate having cavities of minimum equivalent diameter e_(cm) comprises the following successive steps:

a) a cement consisting of spherical particles of an activating agent, of an inert diluent and of a metallic powder is prepared, said spherical particles each having a diameter d such that d≦e_(cm)/10;

b) the cement prepared in step a) is introduced into the cavities of the substrate by a vibrating system;

c) the substrate-cement assembly is heated at a temperature below the melting point of the metallic powder for a duration of at least 6 h at around 650° C. for aluminum;

d) the substrate-cement assembly is cooled to ambient temperature;

e) the cement is subjected to a vibration step so as to eliminate the cement residue;

f) the substrate-cement assembly is heated at a temperature of between 900° C. and 1150° C., preferably above 980° C.; and

g) a substrate having a coating over its entirety is recovered.

By way of example, if the metallic powder is an aluminum powder, in step c) the substrate-cement assembly is heated at around 650° C. for at least 6 h.

Depending on the case, the first process may have one or more of the following features:

the particles of the cement prepared in step a) are preactivated by mechanosynthesis; the preactivation makes it possible to increase the chemical reactivity of the precursor particles. This treatment facilitates the reaction between the precursor and the activator, and therefore the deposition;

the coating recovered in step g) comprises NiAl;

the coating recovered in step g) has a thickness of between 15 and 25 μm.

The second process for depositing a coating by pack cementation on a substrate having cavities of minimum equivalent diameter e_(cm) comprises the following successive steps:

a) a cement consisting of a pickling flux and spherical particles of an inert diluent and of a metallic powder is prepared, said spherical particles each having a diameter d such that d≦e_(cm)/10;

b) the cement prepared in step a) is introduced into the cavities of the substrate by a vibrating system;

c) the substrate-cement assembly is heated at a temperature above the melting point of the pickling flux, under low vacuum or under an inert atmosphere (Ar), for a duration of between 10 min and 2 h;

d) the substrate-cement assembly is cooled to ambient temperature;

e) the cement is subjected to a washing step so as to eliminate the cement residue;

f) a substrate having a coating over its entirety is recovered.

Depending on the case, the second process may have one or more of the following features:

the washing step e) is carried out using an acidified aqueous solution;

the coating recovered in step f) comprises NiAl₃;

said process comprises, before step e), a step of heating the substrate-cement assembly at a temperature of between 900° C. and 1150° C., preferably above 980° C.;

the coating recovered in step f) comprises NiAl;

the coating recovered in step f) has a thickness of between 5 μm and 200 μm, preferably between 5 μm and 80 μm.

FIG. 1 schematically shows the various steps of the first process according to the invention.

The first process consists of the use of a pulverulent mixture consisting of the activating agent (5%), of an inert diluent (alumina, silica, etc.) and of a metal to be deposited, a metallic powder (between 10% and 60%) which may be either pure aluminum or an Al+NiAl or AlCr mixture and the particles of which may or may not have been “preactivated” by mechanosynthesis.

The particle size of the mixture is then adjusted so that it can be introduced into the channels by a vibrating system. The assembly is then brought to a temperature below the melting point of the metal to be deposited for a duration of at least 6 h.

After cooling, the assembly is again subjected to a vibration step that enables the extraction of the residual powder. At this stage, the coating consists of a surface enrichment in aluminum of the substrate, the composition of which is close to NiAl₃. The thicknesses obtained vary between 5 and 10 μm depending on the time for which the first heating step was carried out. After this step, the part thus coated is brought to a temperature of between 900° C. and 1150° C., preferably above 980° C. so as to obtain the composition NiAl in a surface border having a thickness ranging from 15 to 25 μm (FIG. 3).

FIG. 2 schematically shows the various steps of the second process according to the invention.

The second process consists of the use of a pulverulent mixture consisting of a low melting point pickling flux (K₃AlF₆-KAlF₄), this is the element that has the lowest melting point of the mixture constituting the cement and particles, of an inert diluent and of a pure or aluminum alloy metallic powder (10% to 60% of metallic powders, 40% of pickling flux and the balance of inert diluent).

Everything is introduced by vibration as in the case of the first process and is brought to high temperature, below the melting point of the metallic phase, but above that of the pickling flux for a time that varies from a few minutes to one or two hours.

It should be noted that the coating is obtained either under low vacuum or under a controlled inert (argon) atmosphere.

The residues are then extracted by washing directly after the heat treatment step. In order to further improve the extraction of the residues, the apparatus may be washed with a chemical (acidified aqueous) solution. The coating thus obtained corresponds to a phase having a composition close to NiAl₃ which may be converted into NiAl during a subsequent annealing step at a temperature of between 900° C. and 1150° C., preferably at 980° C. The appearance of the coating is shown in FIG. 4.

The mixtures of powders may be stored over long durations in a desiccator under low vacuum or even in a dry chamber under inert gas flushing and are immediately ready to use.

Preferably, the inert diluent is selected from powders of refractory inert materials, more preferably from refractory mineral oxides, such as alumina, silica, magnesia and mixtures thereof, which are commonly used in pack cementation treatments.

The substrate which may be provided with such a coating is generally selected from metallic substrates, for example based on iron or nickel, substrates made of alloy(s) or made of superalloy(s), composite substrates comprising one or more metals and/or alloy(s) and/or superalloy(s) containing Ni in order to react with the Al deposited and form NiAl.

Depending on the desired coating thickness, the substrate could be surface-enriched with Ni beforehand, for example by electrolytic deposition.

As examples of parts on which it is possible to carry out the deposition processes according to the invention, mention may be made of the inside of tubes, turbine blades, heat exchangers, in particular metallic heat exchangers, reactor exchangers, storage vessels, etc.

The treatments are generally carried out under an inert or reducing atmosphere, for example under a hydrogen and/or argon atmosphere, preferably under an argon atmosphere, or else under an argon atmosphere with for example 5% to 10% of hydrogen.

The pressure used during the treatment may be atmospheric pressure or a reduced pressure, for example a pressure of 10⁻² atm of argon.

The coatings obtained by the processes according to the invention give the substrates excellent resistance to corrosion, even within each substrate cavity independently of its size.

Consequently, the service life of these substrates is substantially improved.

EXAMPLE

The photos from FIG. 3 show two samples of HR120 alloy, one (the one on the right) coated with the coating produced according to the second process according to the invention and the other (the one on the left) being uncoated. These samples were subjected to a corrosive atmosphere consisting of (in vol %): 15% CO, 5% CO₂, 55% H₂, 25% H₂O, at a pressure of 21 bar absolute and a temperature of 650° C. After 4700 hours of exposure, it clearly appears that the coating deposited according to the second process of the invention makes it possible to protect the alloy from corrosion. 

1-19. (canceled)
 20. The use of a cement in a process for pack cementation deposition on a substrate having cavities of minimum equivalent diameter e_(cm), characterized in that the cement consists of spherical particles each having a diameter d such that d≦e_(cm)/10.
 21. The use of claim 20, wherein at least some of the spherical particles are comprised of a precursor of the element to be deposited, at least some of the spherical particles are comprised of an activating agent and some of the spherical particles are comprised of an inert diluent.
 22. The use of claim 21, wherein: the precursor of the element to be deposited comprises a metallic powder; 10% to 60% of the spherical particles are comprised of the metallic powder; 5% to 40% of the spherical particles are comprised of the activating agent, and a balance of the spherical particles are comprised of the inert diluent.
 23. The use of claim 22, wherein the metallic powder consists of aluminum or a mixture of aluminum with Ni_(x)Al_(y) particles or Al_(x)Cr_(y)particles.
 24. The use of claim 20, wherein: at least some of the spherical particles are comprised of a precursor of the element to be deposited; at least some of the spherical particles are comprised of a pickling flux; and at least some of the spherical particles are comprised of an inert diluent.
 25. The use of claim 20, wherein at least some of the spherical particles are comprised of an organic or inorganic binder.
 26. The use of claim 20, wherein the substrate is a metallic heat exchanger.
 27. A process for depositing a coating by pack cementation on a substrate having cavities of minimum equivalent diameter e_(cm), comprising the following successive steps: a) preparing a cement consisting of three types of spherical particles, a first of the three types being comprised of an activating agent, a second of the three types being comprised of an inert diluents, a third of the three types being comprised of a metallic powder, each of said three types of spherical particles having a diameter d such that d≦e_(cm)/10; b) the prepared cement is introduced into the cavities of the substrate by a vibrating system to produce a substrate-cement assembly; c) the produced substrate-cement assembly is heated at a temperature below a melting point of the metallic powder for a duration of at least 6 h at around 650° C. for aluminum; d) the substrate-cement assembly is cooled to ambient temperature; e) the cement is subjected to a vibration step so as to eliminate the cement residue; f) the substrate-cement assembly is heated at a temperature of between 900° C. and 1150° C., preferably above 980° C.; and g) a substrate having a coating over its entirety is recovered.
 28. The process of claim 27, wherein the metal of the metallic powder is aluminum.
 29. The process of claim 28, wherein the temperature, at which the produced substrate-cement assembly is heated, is around 650° C.
 30. The process of claim 27, wherein the temperature, at which the substrate-cement assembly is heated, is between 900° C. and 1150° C.
 31. The process of claim 30, wherein the temperature, at which the substrate-cement assembly is heated, is above 980° C.
 32. The deposition process of claim 27, wherein the particles of the cement prepared in step a) are preactivated by mechanosynthesis.
 33. The deposition process of claim 27, wherein the coating recovered in step g) comprises NiAl.
 34. The deposition process of claim 27, wherein the coating recovered in step g) has a thickness of between 15 and 25 μm.
 35. The process of claim 27, wherein the substrate is a metallic heat exchanger.
 36. A process for depositing a coating by pack cementation on a substrate having cavities of minimum equivalent diameter e_(cm), comprising the following successive steps: a) preparing a cement consisting of a pickling flux, spherical particles of an inert diluent and spherical particles of a metallic powder, each of the spherical particles of inert diluents and metallic powder having a diameter d such that d≦e_(cm)/10; b) introducing the prepared cement into the cavities of the substrate by a vibrating system to prepare substrate-cement; c) heating the prepared substrate-cement assembly to a temperature above a melting point of the pickling flux, under either a low vacuum or under an inert atmosphere (Ar), for a duration of between 10 min and 2 hr, to thereby produce a substrate-cement assembly having a coating of cement residue; d) cooling the coated substrate-cement assembly to ambient temperature; e) subjecting the coated substrate-cement assembly to a washing step so as to eliminate a cement residue such that an entirety of the coating is recovered from the substrate-cement assembly.
 37. The process of claim 36, wherein the washing step e) is carried out using an acidified aqueous solution.
 38. The process of claim 36, wherein the coating recovered in step f) comprises NiAl₃.
 39. The process of claim 36, wherein said process comprises, before step e) and after step d), a step of heating the coated substrate-cement assembly at a temperature of between 900° C. and 1150° C.
 40. The process of claim 39, wherein the coated substrate-cement assembly is heated to a temperature above 980° C.
 41. The process of claim 39, wherein the coating recovered in step f) comprises NiAl.
 42. The process of claim 36, wherein the coating recovered in step f) has a thickness of between 5 μm and 200 μm, preferably between 5 μm and 80 μm. 